Eleven people fell ill on Tuesday after a suspicious letter was opened at a US military base in Arlington, Virginia, the fire department and a base spokeswoman said. “Someone opened a letter and everyone at the office started feeling badly,” said Leah Rubalcaba, a spokeswoman for Joint Base Myer-Henderson Hall. The three people taken to a hospital were in stable condition, Arlington Fire Department said on Twitter. The letter was opened in the Marine Corps’ portion of the base, which is located near Washington. The joint base is across the Potomac River from Washington, D.C., bordering Arlington National Cemetery. “An envelope containing an unknown substance was received” at the base, the Marine Corps said in a statement. The Marine Corps said the building was evacuated, and several Marines are receiving medical care. The Corps said the Naval Criminal Investigative Service and the FBI are conducting a joint investigation.